There are many applications wherein it is necessary to store energy in capacitive or inductive loads and then release the stored energy quickly. One such application is in the ignition system of an internal combustion engine. Here, energy is stored in an ignition coil during a dwell period. At the end of the dwell period the stored energy is quickly released or discharged across a spark gap of a spark plug. To control the storage and release of the energy, ignition control circuits are provided. To reduce the size and cost of these circuits, ignition control circuits are generally provided in integrated circuit form and include an output transistor which controls an external power switch, such as a power transistor or transistors, which are disposed in series with the primary of the ignition coil between a voltage source and ground potential.
During the dwell period, the ignition circuit output transistor turns the power switch on to permit current flow through the ignition coil primary. At the end of the dwell period, the output transistor turns the power switch off to cause the stored energy to be released across the spark gap through the ignition coil secondary. During the release of the stored energy, the spark extinguishes before all of the stored energy is totally released. The residual stored energy therefore creates a large negative voltage across the ignition coil primary which can be propagated back to the output transistor. When this occurs, the output transistor can be inadvertently forward biased into saturation pulling the emitter and collector thereof down to below ground potential. If the output transistor is formed on the integrated circuit, and is isolated from the other components thereof by, for example, a grounded p-type substrate and isolation layers, then the negative potential on the collector can cause integrated circuit substrate injection by forward biasing the junction between the isolation and the collector. This in essence removes the isolation between the integrated circuit components and adversely effects its operation.
To overcome this problem in the prior art, the output transistor has been formed to be a large PNP transistor. While this has generally solved the problem, these PNP output transistors are made large and thus take up valuable integrated circuit area. Another attempt has been to leave the output transistor off of the integrated circuit and thereby make it an external component. However, this adds to part count which increases the cost of such a system. In summary, there is a need in the art for an ignition control integrated circuit which both includes the output transistor on the integrated circuit and which includes means for preventing substrate injection without resorting to large internal PNP transistors.
It is therefore a general object of the present invention to provide an integrated circuit adapted to facilitate storage of energy in and the release of energy from external capacitive or inductive loads which includes means for preventing substrate injection within the integrated circuit during the release of the stored energy.
It is a more particular object of the present invention to provide an ignition control integrated circuit including an output transistor which controls the storage of energy in and the release of energy from an inductive load and which includes means for preventing current flow through the base of the output transistor during the release of the stored energy.
It is a further object of the present invention to provide such an integrated circuit wherein the output transistor is an NPN transistor.